A polymer electrolyte fuel cell simultaneously generates electricity and heat by electrochemically reacting a fuel gas such as hydrogen and an oxidant gas such as air in a gas diffusion electrode having a catalyst layer such as platinum with use of a polymer electrolyte membrane, which selectively transfers hydrogen ions, as an electrolyte.
A catalytic reaction layer mainly composed of a carbon powder carrying a platinum group metal catalyst and a polymer electrolyte covering the carbon powder is tightly attached to both surfaces of the polymer electrolyte membrane. On the outer face of the catalyst layers is tightly attached a pair of gas diffusion layers having gas permeability and electroconductivity. The gas diffusion layer and the catalytic reaction layer constitute a gas diffusion electrode. On the outer face of the gas diffusion electrode is disposed a conductive separator plate for mechanically fastening an electrode electrolyte membrane assembly comprising the polymer electrolyte membrane and the gas diffusion layer (hereinafter referred to as “MEA”), and at the same time for electrically interconnecting adjacent MEAs in series. The conductive separator plate has gas channels for supplying a reactant gas to the electrodes, and for conveying a surplus gas and water generated by a reaction. Although the gas channels may be arranged separately from the separator plate, grooves are usually formed on the surface of the separator plate to serve as gas channels.
On the other face of the separator plate are provided cooling channels for circulating cooling water so as to maintain the cell temperature constant. Due to the circulation of cooling water, a thermal energy generated by a reaction can be used in the form of hot water or the like. In order to supply and discharge such reactant gas and cooling water, channels called manifolds are provided.
Gaskets with sealing capability are arranged on the periphery of the electrodes to prevent gas leakage to the counter electrode or to the outside. Typically, an end plate is arranged at each end of a stack, in which a plurality of unit cells are stacked in one direction, and the two end plates are fixed with a clamping member in order to permanently clamp the whole stack.
The gas diffusion electrode of this type of fuel cell mainly has three functions described below. The first function is to diffuse the reactant gas to uniformly supply the fuel gas or oxidant gas from gas channels formed on the outer face of the gas diffusion layer to a catalyst within the catalyst layer. The second function is to rapidly discharge water produced by a reaction in the catalyst layer to the gas channels. The third function is to conduct electrons required or produced by a reaction.
Accordingly, the gas diffusion layer is required to have high gas permeability, high water discharge capability and high electronical conductivity. As a conventional technique for imparting gas permeability, the substrate for gas diffusion layer is designed to have a porous structure. For imparting water discharge capability, a water repellent polymer such as fluorocarbon resin is dispersed within the gas diffusion layer, thereby preventing water clogging (flooding). For imparting electronical conductivity, the gas diffusion layer is made using an electronically conductive material such as carbon fiber, metal fiber and carbon fine powder.
There are mainly three types of gas diffusion layers currently in use. One is what is called carbon paper. The second is what is called carbon cloth (fabric). The third is what is called carbon felt (non-woven fabric).
Carbon paper is carbon fibers made into the form of a sheet and has proper rigidity. It is usually used in a fuel cell whose stack is clamped with a predetermined clamping pressure because the substrate does not fall down into the gas channels of the separator plate and it is easy to be designed due to low pressure-loss. It is, however, difficult to control porosity, and flooding is likely to occur because carbon fibers are randomly oriented. Moreover, since carbon paper is in the form of a sheet, it is difficult to form it into a roll and its continuous production is also difficult, facing the challenge of reducing production cost.
As for carbon cloth, since the fibers are oriented in a particular direction, it is relatively easy to control porosity. Accordingly, flooding is unlikely to occur during the supply of highly humidified reactant gas in the operation. Furthermore, since carbon cloth is a fabric, it is flexible and therefore continuous production can be achieved, thus offering an advantage in terms of production cost. Carbon felt is considered to be promising because its production cost is obviously the lowest. It has, however, the problem of reliability at present, and a rapid improvement in reliability is now expected to be made. In view of these points, carbon cloth is currently superior in terms of reliability, mass productivity and performance in a high humidity environment.
The use of carbon cloth as the substrate for gas diffusion layer for fuel cell has the above-described advantages; at the same time, it has many disadvantages. Since carbon cloth is a fabric, it has concave and convex portions in the thickness direction on the surface thereof, which are unavoidable. In a polymer electrolyte fuel cell intended to be thin in order to achieve higher performance, the carbon fibers of carbon cloth pierce the polymer electrolyte membrane to generate a micro short-circuit, thereby causing the lowering of voltage. Further, this micro short-circuit causes hydrogen and oxygen to react with each other in an electrode to cause a combustion reaction, which also lowers reliability due to the degradation of polymer electrolyte membrane.
To cope with these problems, in a technique described in one prior art, Japanese Laid-Open Patent Publication No. Hei 10-261421, when a water repellent layer is formed on a carbon cloth, the carbon cloth is impregnated with a part of the water repellent layer from the surface of the carbon cloth and into the carbon cloth. It is not allowed to impregnate the whole carbon cloth with the water repellent layer. In other words, it is a technique for smoothing the concave and convex portions of the carbon cloth by the water repellent layer on the surface of the carbon cloth. This prior art reference, however, does not teach the technical idea of preventing the micro short-circuit as described above.
Moreover, another prior art, Japanese Laid-Open Patent Publication No. 2001-85019, discloses a technique for preventing the micro short-circuit by forming a water repellent layer on the surface of a carbon cloth, which is then hot-pressed to smooth the concave and convex portions of the carbon cloth.
Any of these prior art references, however, does not cope with a carbon cloth material itself. In other words, the prior art references are intended, not to prevent the surface concave and convex portions of the carbon cloth per se, but to smooth the water repellent layer or to smooth the whole gas diffusion layer substrate including the water repellent layer. Accordingly, these prior art references have limits in smoothing or preventing the micro short-circuit, and the manufacturing process thereof is complicated, which makes the production cost higher.
Furthermore, the micro short-circuit results not only from the concave and convex portions but also from fuzz present on the gas diffusion substrate.